Type Objects

Perhaps one of the most important structures of the Python object system is the structure that defines a new type: the PyTypeObject structure. Type objects can be handled using any of the PyObject_*() or PyType_*() functions, but do not offer much that’s interesting to most Python applications. These objects are fundamental to how objects behave, so they are very important to the interpreter itself and to any extension module that implements new types.

Type objects are fairly large compared to most of the standard types. The reason for the size is that each type object stores a large number of values, mostly C function pointers, each of which implements a small part of the type’s functionality. The fields of the type object are examined in detail in this section. The fields will be described in the order in which they occur in the structure.

Typedefs: unaryfunc, binaryfunc, ternaryfunc, inquiry, coercion, intargfunc, intintargfunc, intobjargproc, intintobjargproc, objobjargproc, destructor, freefunc, printfunc, getattrfunc, getattrofunc, setattrfunc, setattrofunc, cmpfunc, reprfunc, hashfunc

The structure definition for PyTypeObject can be found in Include/object.h. For convenience of reference, this repeats the definition found there:

typedef struct _typeobject {
    PyObject_VAR_HEAD
    char *tp_name; /* For printing, in format "<module>.<name>" */
    int tp_basicsize, tp_itemsize; /* For allocation */

    /* Methods to implement standard operations */

    destructor tp_dealloc;
    printfunc tp_print;
    getattrfunc tp_getattr;
    setattrfunc tp_setattr;
    cmpfunc tp_compare;
    reprfunc tp_repr;

    /* Method suites for standard classes */

    PyNumberMethods *tp_as_number;
    PySequenceMethods *tp_as_sequence;
    PyMappingMethods *tp_as_mapping;

    /* More standard operations (here for binary compatibility) */

    hashfunc tp_hash;
    ternaryfunc tp_call;
    reprfunc tp_str;
    getattrofunc tp_getattro;
    setattrofunc tp_setattro;

    /* Functions to access object as input/output buffer */
    PyBufferProcs *tp_as_buffer;

    /* Flags to define presence of optional/expanded features */
    long tp_flags;

    char *tp_doc; /* Documentation string */

    /* Assigned meaning in release 2.0 */
    /* call function for all accessible objects */
    traverseproc tp_traverse;

    /* delete references to contained objects */
    inquiry tp_clear;

    /* Assigned meaning in release 2.1 */
    /* rich comparisons */
    richcmpfunc tp_richcompare;

    /* weak reference enabler */
    long tp_weaklistoffset;

    /* Added in release 2.2 */
    /* Iterators */
    getiterfunc tp_iter;
    iternextfunc tp_iternext;

    /* Attribute descriptor and subclassing stuff */
    struct PyMethodDef *tp_methods;
    struct PyMemberDef *tp_members;
    struct PyGetSetDef *tp_getset;
    struct _typeobject *tp_base;
    PyObject *tp_dict;
    descrgetfunc tp_descr_get;
    descrsetfunc tp_descr_set;
    long tp_dictoffset;
    initproc tp_init;
    allocfunc tp_alloc;
    newfunc tp_new;
    freefunc tp_free; /* Low-level free-memory routine */
    inquiry tp_is_gc; /* For PyObject_IS_GC */
    PyObject *tp_bases;
    PyObject *tp_mro; /* method resolution order */
    PyObject *tp_cache;
    PyObject *tp_subclasses;
    PyObject *tp_weaklist;

} PyTypeObject;

The type object structure extends the PyVarObject structure. The ob_size field is used for dynamic types (created by type_new(), usually called from a class statement). Note that PyType_Type (the metatype) initializes tp_itemsize, which means that its instances (i.e. type objects) must have the ob_size field.

PyObject* PyObject._ob_next

PyObject* PyObject._ob_prev

These fields are only present when the macro Py_TRACE_REFS is defined. Their initialization to NULL is taken care of by the PyObject_HEAD_INIT macro. For statically allocated objects, these fields always remain NULL. For dynamically allocated objects, these two fields are used to link the object into a doubly-linked list of all live objects on the heap. This could be used for various debugging purposes; currently the only use is to print the objects that are still alive at the end of a run when the environment variable PYTHONDUMPREFS is set.

These fields are not inherited by subtypes.

Py_ssize_t PyObject.ob_refcnt

This is the type object’s reference count, initialized to 1 by the PyObject_HEAD_INIT macro. Note that for statically allocated type objects, the type’s instances (objects whose ob_type points back to the type) do not count as references. But for dynamically allocated type objects, the instances do count as references.

This field is not inherited by subtypes.

Changed in version 2.5: This field used to be an int type. This might require changes in your code for properly supporting 64-bit systems.

PyTypeObject* PyObject.ob_type

This is the type’s type, in other words its metatype. It is initialized by the argument to the PyObject_HEAD_INIT macro, and its value should normally be &PyType_Type. However, for dynamically loadable extension modules that must be usable on Windows (at least), the compiler complains that this is not a valid initializer. Therefore, the convention is to pass NULL to the PyObject_HEAD_INIT macro and to initialize this field explicitly at the start of the module’s initialization function, before doing anything else. This is typically done like this:

Foo_Type.ob_type = &PyType_Type;

This should be done before any instances of the type are created. PyType_Ready() checks if ob_type is NULL, and if so, initializes it: in Python 2.2, it is set to &PyType_Type; in Python 2.2.1 and later it is initialized to the ob_type field of the base class. PyType_Ready() will not change this field if it is non-zero.

In Python 2.2, this field is not inherited by subtypes. In 2.2.1, and in 2.3 and beyond, it is inherited by subtypes.

Py_ssize_t PyVarObject.ob_size

For statically allocated type objects, this should be initialized to zero. For dynamically allocated type objects, this field has a special internal meaning.

This field is not inherited by subtypes.

char* PyTypeObject.tp_name

Pointer to a NUL-terminated string containing the name of the type. For types that are accessible as module globals, the string should be the full module name, followed by a dot, followed by the type name; for built-in types, it should be just the type name. If the module is a submodule of a package, the full package name is part of the full module name. For example, a type named T defined in module M in subpackage Q in package P should have the tp_name initializer "P.Q.M.T".

For dynamically allocated type objects, this should just be the type name, and the module name explicitly stored in the type dict as the value for key '__module__'.

For statically allocated type objects, the tp_name field should contain a dot. Everything before the last dot is made accessible as the __module__ attribute, and everything after the last dot is made accessible as the __name__ attribute.

If no dot is present, the entire tp_name field is made accessible as the __name__ attribute, and the __module__ attribute is undefined (unless explicitly set in the dictionary, as explained above). This means your type will be impossible to pickle. Additionally, it will not be listed in module documentations created with pydoc.

This field is not inherited by subtypes.

Py_ssize_t PyTypeObject.tp_basicsize

Py_ssize_t PyTypeObject.tp_itemsize

These fields allow calculating the size in bytes of instances of the type.

There are two kinds of types: types with fixed-length instances have a zero tp_itemsize field, types with variable-length instances have a non-zero tp_itemsize field. For a type with fixed-length instances, all instances have the same size, given in tp_basicsize.

For a type with variable-length instances, the instances must have an ob_size field, and the instance size is tp_basicsize plus N times tp_itemsize, where N is the “length” of the object. The value of N is typically stored in the instance’s ob_size field. There are exceptions: for example, long ints use a negative ob_size to indicate a negative number, and N is abs(ob_size) there. Also, the presence of an ob_size field in the instance layout doesn’t mean that the instance structure is variable-length (for example, the structure for the list type has fixed-length instances, yet those instances have a meaningful ob_size field).

The basic size includes the fields in the instance declared by the macro PyObject_HEAD or PyObject_VAR_HEAD (whichever is used to declare the instance struct) and this in turn includes the _ob_prev and _ob_next fields if they are present. This means that the only correct way to get an initializer for the tp_basicsize is to use the sizeof operator on the struct used to declare the instance layout. The basic size does not include the GC header size (this is new in Python 2.2; in 2.1 and 2.0, the GC header size was included in tp_basicsize).

These fields are inherited separately by subtypes. If the base type has a non-zero tp_itemsize, it is generally not safe to set tp_itemsize to a different non-zero value in a subtype (though this depends on the implementation of the base type).

A note about alignment: if the variable items require a particular alignment, this should be taken care of by the value of tp_basicsize. Example: suppose a type implements an array of double. tp_itemsize is sizeof(double). It is the programmer’s responsibility that tp_basicsize is a multiple of sizeof(double) (assuming this is the alignment requirement for double).

destructor PyTypeObject.tp_dealloc

A pointer to the instance destructor function. This function must be defined unless the type guarantees that its instances will never be deallocated (as is the case for the singletons None and Ellipsis).

The destructor function is called by the Py_DECREF() and Py_XDECREF() macros when the new reference count is zero. At this point, the instance is still in existence, but there are no references to it. The destructor function should free all references which the instance owns, free all memory buffers owned by the instance (using the freeing function corresponding to the allocation function used to allocate the buffer), and finally (as its last action) call the type’s tp_free function. If the type is not subtypable (doesn’t have the Py_TPFLAGS_BASETYPE flag bit set), it is permissible to call the object deallocator directly instead of via tp_free. The object deallocator should be the one used to allocate the instance; this is normally PyObject_Del() if the instance was allocated using PyObject_New() or PyObject_VarNew(), or PyObject_GC_Del() if the instance was allocated using PyObject_GC_New() or PyObject_GC_NewVar().

This field is inherited by subtypes.

printfunc PyTypeObject.tp_print

An optional pointer to the instance print function.

The print function is only called when the instance is printed to a real file; when it is printed to a pseudo-file (like a StringIO instance), the instance’s tp_repr or tp_str function is called to convert it to a string. These are also called when the type’s tp_print field is NULL. A type should never implement tp_print in a way that produces different output than tp_repr or tp_str would.

The print function is called with the same signature as PyObject_Print(): int tp_print(PyObject *self, FILE *file, int flags). The self argument is the instance to be printed. The file argument is the stdio file to which it is to be printed. The flags argument is composed of flag bits. The only flag bit currently defined is Py_PRINT_RAW. When the Py_PRINT_RAW flag bit is set, the instance should be printed the same way as tp_str would format it; when the Py_PRINT_RAW flag bit is clear, the instance should be printed the same was as tp_repr would format it. It should return -1 and set an exception condition when an error occurred during the comparison.

It is possible that the tp_print field will be deprecated. In any case, it is recommended not to define tp_print, but instead to rely on tp_repr and tp_str for printing.

This field is inherited by subtypes.

getattrfunc PyTypeObject.tp_getattr

An optional pointer to the get-attribute-string function.

This field is deprecated. When it is defined, it should point to a function that acts the same as the tp_getattro function, but taking a C string instead of a Python string object to give the attribute name. The signature is

PyObject * tp_getattr(PyObject *o, char *attr_name);

This field is inherited by subtypes together with tp_getattro: a subtype inherits both tp_getattr and tp_getattro from its base type when the subtype’s tp_getattr and tp_getattro are both NULL.

setattrfunc PyTypeObject.tp_setattr

An optional pointer to the function for setting and deleting attributes.

This field is deprecated. When it is defined, it should point to a function that acts the same as the tp_setattro function, but taking a C string instead of a Python string object to give the attribute name. The signature is

PyObject * tp_setattr(PyObject *o, char *attr_name, PyObject *v);

The v argument is set to NULL to delete the attribute. This field is inherited by subtypes together with tp_setattro: a subtype inherits both tp_setattr and tp_setattro from its base type when the subtype’s tp_setattr and tp_setattro are both NULL.

cmpfunc PyTypeObject.tp_compare

An optional pointer to the three-way comparison function.

The signature is the same as for PyObject_Compare(). The function should return 1 if self greater than other, 0 if self is equal to other, and -1 if self less than other. It should return -1 and set an exception condition when an error occurred during the comparison.

This field is inherited by subtypes together with tp_richcompare and tp_hash: a subtypes inherits all three of tp_compare, tp_richcompare, and tp_hash when the subtype’s tp_compare, tp_richcompare, and tp_hash are all NULL.

reprfunc PyTypeObject.tp_repr

An optional pointer to a function that implements the built-in function repr().

The signature is the same as for PyObject_Repr(); it must return a string or a Unicode object. Ideally, this function should return a string that, when passed to eval(), given a suitable environment, returns an object with the same value. If this is not feasible, it should return a string starting with '<' and ending with '>' from which both the type and the value of the object can be deduced.

When this field is not set, a string of the form <%s object at %p> is returned, where %s is replaced by the type name, and %p by the object’s memory address.

This field is inherited by subtypes.

PyNumberMethods* tp_as_number

Pointer to an additional structure that contains fields relevant only to objects which implement the number protocol. These fields are documented in Number Object Structures.

The tp_as_number field is not inherited, but the contained fields are inherited individually.

PySequenceMethods* tp_as_sequence

Pointer to an additional structure that contains fields relevant only to objects which implement the sequence protocol. These fields are documented in Sequence Object Structures.

The tp_as_sequence field is not inherited, but the contained fields are inherited individually.

PyMappingMethods* tp_as_mapping

Pointer to an additional structure that contains fields relevant only to objects which implement the mapping protocol. These fields are documented in Mapping Object Structures.

The tp_as_mapping field is not inherited, but the contained fields are inherited individually.

hashfunc PyTypeObject.tp_hash

An optional pointer to a function that implements the built-in function hash().

The signature is the same as for PyObject_Hash(); it must return a C long. The value -1 should not be returned as a normal return value; when an error occurs during the computation of the hash value, the function should set an exception and return -1.

This field can be set explicitly to PyObject_HashNotImplemented() to block inheritance of the hash method from a parent type. This is interpreted as the equivalent of __hash__ = None at the Python level, causing isinstance(o, collections.Hashable) to correctly return False. Note that the converse is also true - setting __hash__ = None on a class at the Python level will result in the tp_hash slot being set to PyObject_HashNotImplemented().

When this field is not set, two possibilities exist: if the tp_compare and tp_richcompare fields are both NULL, a default hash value based on the object’s address is returned; otherwise, a TypeError is raised.

This field is inherited by subtypes together with tp_richcompare and tp_compare: a subtypes inherits all three of tp_compare,